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Old 28th July 2011, 04:04 PM   #1
taj is offline taj
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Default Retrofit idea, need advice, 6L6SE guitar amp

I have this little Masco ME-8 amplifier whose circuitry has been universally condemned by the local braintrust, and yesterday I happened to stumble across this schematic in my RCA tube manual, which uses virtually identical chassis hardware, making a retrofit pretty much a no-brainer if this circuit is worthwhile. It certainly seems normal/acceptable to me for use as a guitar amp (ie. linearity, distortion and bandwidth are not really a concern.)

But the ME-8 used octal tubes and this RCA design uses 9-pin tubes, but I believe I can use a 6SL7 in place of the 6EU7, and find a suitable octal version of the 1/2 6SL7 for the other tube. Or perhaps I could incorporate another triode into the circuit somehow, in order to end up just using two 6SL7s.

Can I just parallel the 2 triodes and adjust the operating points accordingly?What does that accomplish? More current flow certainly, but more gain too? Or might there be a better way to incorporate another triode without adding too much complexity (the chassis is really small/tight).

Your thoughts?

..Todd
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Old 28th July 2011, 04:26 PM   #2
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I had a look at your other thread on the Masco. Why don't you just use the Masco as is for a guitar amp? A lot of classic guitar amps had grid lead bias just as the Masco does on the Mic input. If the problem is that you are using stompboxes and the Mic input is saturating from the higher input, then just convert that 1st stage into cathode bias.
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Old 28th July 2011, 04:26 PM   #3
TheGimp is offline TheGimp  United States
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I'd change the first stage (In1) from grid-leak bias to cathode bias with a bypass cap. Remove the 100pf cap and move the series resistor to the grid as a gridstop. And change the tone controls to a more traditional tonestack. THree stages is plenty, but you could make the second bottle a dual triode and either parallel them as you suggest or add an additional gain stage for more overdrive (but you might want to add an additional gain control between stages to give you control over it).
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Old 28th July 2011, 05:34 PM   #4
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Hi TheGimp,

I think adding an additional gain stage would add too many parts for the chassis size -- especially the added pot needed to control it. I don't want to add a pot.

What would you consider a more traditional/suitable tone stack? It would still have to be a passive R/C network though or it probably won't fit (physically). I haven't played guitar in many years so I'm not up on guitar circuitry. (And I'm not exactly a whiz with hi-fi circuitry either.)

..Todd
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Old 28th July 2011, 10:01 PM   #5
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I checked out some tone stacks. This RCA one looks like a Baxandall topology. I've heard good things about those. No?

..todd
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Old 28th July 2011, 10:50 PM   #6
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Compare the schematic of the original Masco with the Fender Champ 5C1. Looks like a no brainer to me. I would fix it and try it first it might just rock! If not rewire it into a Champ using the 6SJ7, 6L6GCF and 5Y3.

If you want a tone control the extra tube can supply the gain needed to make up for a tone stack loss. I would stick to a FMV (Fender Marshall Vox) tone stack since it is optimized for guitar use. A Baxandall tone control is excellent for HiFi but it is rarely used in guitar amps since it is optimized for action outside the guitars fundamental frequency range.

I have seen a few guitar amp schematics that use a pentode first stage followed by the tone stack then a triode then the SE output tube, but I can't find one now.

Champ 5C1:
http://www.drtube.com/schematics/fen...-5c1-schem.gif
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Old 28th July 2011, 11:32 PM   #7
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George... that's perfect. Thanks!

I'll try it as-is first. ...well, after I refurb it slightly.

..Todd
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Old 28th July 2011, 11:52 PM   #8
Cassiel is offline Cassiel  Libya
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Quote:
I'll try it as-is first.
The local feedback resistor (R13) takes away gain and distortion. It would probably sound better without it.
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Old 28th July 2011, 11:53 PM   #9
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Quote:
Originally Posted by Cassiel View Post
The local feedback resistor (R13) takes away gain and distortion. It would probably sound better without it.
Good point. thanks.
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Old 29th July 2011, 02:08 AM   #10
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Grid leak bias is preferred for a harp amp, try it! But it won't work well with a high-level input like a pedal. The Fender tone stack is NOT flat - it has a notch in the low midrange that makes a guitar sound MUCH better. A Baxendall can be made with the proper roll-of frequencies for guitar, but won't sound good when set "flat".
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